In some disciplines (such as theoretical computer science), it is typical to have papers where you have co-authors. Especially when the set of co-authors is large, and work is being done at different universities, it is possible that person X works on one part, and person Y on one part. Thus, perhaps you wrote and came up with one result, and another result was the result of the work of someone else. Finally, you publish that paper together.
Then, in your PhD thesis, you typically have a list of publications. From what I've seen, the author usually declares what is his/her contribution was for each publication, but this is quite vague. For instance, it might say e.g., "Publication 1 is joint work with X and Y, and partly written by the author".
So consider a particular result that appears in a paper where you are a co-author as a PhD student. My question(s) are:
- Is it ethical to include that result in your PhD thesis, supposing that you did not contribute to the result? Perhaps you only proof-read the theorem.
- Does it make a difference if you couldn't have come up with the result, or don't deeply understand it? Maybe professor Bigshot Superstar came up with it, wrote it, and it is using advanced machinery with several technical details.
It is nice to just say "this is joint work with Bigshot Superstar" to make yourself look good. I see it being done, but it makes me feel uneasy. On the other hand, while it seems honest, it can potentially hurt you to say "the author did not contribute to Theorem X".